Big Slope on the corner of Kelvingrove Street and Argyle Street has open wi-fi access - and it's quite a nice bar too... and the sunday roast is a good 'un...
The Apple Store in the city centre has open wifi. You can either sit inside or you can usually get a pretty strong signal in the Starbucks across the street.
It's been a while since I've been there, but the Goat on Argyle Street in the West End certainly used to have free wireless for customers.
Any burger king and there is plenty of them about glasgow
If you're near Queen Street Station, then The Junction Bar has free wifi. Just right outside the station entrance at the corner of West George St. and Woodside Way.